1. Field of the Invention
The present invention relates to a photoelectric conversion device, an imaging device and a photosensor.
2. Description of Related Art
Visible light sensors in the related art are generally a device produced by forming a photoelectric conversion site through, for example, formation of PN junction in a semiconductor such as Si. As for a solid-state imaging device, there is widely used a flat light-receiving device where photoelectric conversion sites are two-dimensionally arrayed in a semiconductor to form pixels and a signal generated in each pixel through photoelectric conversion is charge-transferred and read out according to a CCD or CMOS format. The method for realizing a color solid-state imaging device is generally to fabricate a structure where on the light incident surface side of the flat image-receiving device, a color filter transmitting only light at a specific wavelength is disposed for color separation. Particularly, a single-plate sensor in which color filters transmitting blue light, green light and red light, respectively, are regularly disposed on each of two-dimensionally arrayed pixels is well known as a system widely used at present in a digital camera and the like.
In this system, since the color filter transmits only light at a limited wavelength, untransmitted light is not utilized and the light utilization efficiency is bad. Also, in recent years, amid the advance in the fabrication of a multipixel device, the pixel size and in turn, the area of a photodiode part becomes small and this brings about problems of reduction in the aperture ratio and reduction in the light collection efficiency.
In order to solve these problems, there may be thought out a system where photoelectric conversion parts capable of detecting light at different wavelengths are stacked in a longitudinal direction. As regards such a system, for example, U.S. Pat. No. 5,965,875 discloses a sensor utilizing wavelength dependency of the absorption coefficient of Si, where a vertical stacked structure is formed and the colors are separated by the difference in the depth, and JP-A-2003-332551 discloses a sensor by a stacked structure using an organic photoelectric conversion layer. However, color separation by the difference in the depth direction of Si is disadvantageously poor because the absorption range is overlapped among respective portions and the spectroscopic property is bad. As for other methods to solve the problems, a structure where a photoelectric conversion layer by amorphous silicon or an organic photoelectric conversion layer is formed on a signal reading substrate is known as a technique for increasing the aperture ratio.
Heretofore, several examples have been known for a photoelectric conversion device, an imaging device and a photosensor each using an organic photoelectric conversion layer. A high photoelectric conversion efficiency (exciton dissociation efficiency, charge transportability) and a low dark current (amount of carrier at dark time) are a problem in particular and for the improvement in this respect, there are disclosed, for example, introduction of pn junction or introduction of a bulk heterostructure for the former and introduction of a blocking layer for the latter.
These improvement methods by structure may produce a large effect, but the properties of the material used also greatly contribute to the device performance. There is almost no report or patent publication regarding the optical response speed which is another important parameter of the organic photoelectric conversion device (in particular, when applied as an imaging device or a photosensor). On the other hand, as regards a photocurrent multiplication device using an organic material (semiconductor), specific organic pigments are described in JP-A-2003-110132 and JP-A-2003-282934, but the response speed exhibited by such a photocurrent for the start of light irradiation (light-on) and the stop of light irradiation (light-off) is as slow as on the order of seconds (milli-seconds at shortest), failing in satisfying a high response speed.
In use as a solid-state imaging device, all of high photoelectric conversion efficiency, low dark current and high response speed need to be satisfied, but there has not been specifically disclosed what an organic photoelectric conversion material or a device structure satisfies this requirement.